This invention relates to electro-absorption optical modulators, particularly to the type of such modulator that incorporate reverse-biased multiquantum well (MQW) structures, and is concerned with saturation effects liable to occur in such modulators.
For the purposes of this specification, a distinction is drawn between a multi-quantum well (MQW) structure of interleaved quantum well layers and barrier layers, and a superlattice structure, similarly of interleaved quantum well layers and barrier layers. This distinction is that, in an MQW structure, the barrier layers are thick enough to preclude any substantial band structure interaction between adjacent quantum well layers; whereas, in a superlattice structure, the barrier layers are specifically and deliberately thin enough to provide interaction sufficient to produce a mini-band structure.
Saturation effects observed in MQW electro-absorption modulators result from the slow escape of photon-generated carriers from the reverse-biased quantum wells. It is believed that these effects are liable to be more pronounced in semiconductor systems, such as InGaAsP/InP and InGaAs/Inp systems, in which the valence band steps on both sides of the quantum well layers are significantly larger than the corresponding conduction band steps. A paper by B W Takasawi et al, entitled "Observation of Separate Electron and Hole Escape Rates in Unbiased Strained InGaAsP Multiple Quantum Well Laser Structures", Applied Physics Letters, 62 (20), 17 May 1993 pp 2525-7, describes measurements made to assess the escape times of photo-generated electrons and holes generated in a multiquantum well structure comprising quantum wells made of InGaAsP interleaved with barrier layers made of InP. The analysis shows that, in this InGaAsP/InP semiconductor system, the hole escape times are significantly longer than the corresponding electron escape times, but that the hole escape times can be reduced from about 18 ns, in the case of examples with unstrained quantum wells, to about 10 ns in the case of examples where the quantum well composition is chosen to put the quantum wells in 1.2% compression, and to about 13 ns in the case of examples where the quantum well composition is instead chosen to put the quantum wells in tension. These reductions in hole escape times, which are attributed to thermally assisted tunnelling effects via higher energy levels (a light hole in the case of compressively strained quantum wells, and a heavy hole in the case of those that are in tensile strain), will clearly have an effect in ameliorating saturation problems to some extent, but still leaves a hole escape time that is inconveniently long for many applications.